Frictional Discrepancies

[writchie]

Originally Posted by AndrewN

Doing an incline plane test with four wheels and a frame we found almost no difference between the lateral and transverse static COF as measured by the angle of the incline. We are using the correct surface too.

Just to make it clear:

1. Point the locked wheels down the incline and gradually raise one end of the surface until the frame breaks free slides down. Measure the height at which the frame breaks free (8.5" over a 6ft sheet). This is a measure of the lateral static COF (tan of the angle between the horizontal and the incline). Our result is around 0.12 or an angle of 6.7 degrees.

2. Turn the frame 90 degrees. The wheels are now sideways down the slope. Repeat test. This is the transverse static COF.
Our result 8.5 - 9". Almost the same as the lateral value.

We expected that the heights of the two tests should almost be a factor of two or more different given 0.6 and 1.4 as the printed static COFs.

Can other teams please repeat this test and report the angles they are finding for both lateral and transverse static friction.

Perhaps you have discovered what's behind the "fish" clue

The transverse/inline ratio is a very critical parameter. If it's no where near the 2.3 advertised, then many preliminary design decisions about drive configuration will be dead wrong. We will try to confirm your findings as soon as we can locate the actual surface material.

It is possible that the ratio changes significantly with normal forces closer to 1/4 of the nominal weight of the robot due to the way the materials deform under load. It could also be that the type of backing underneath the regolith is a factor. The wheels are very hard and provide a very small contact area. If the backing is carpet (rather than a very hard material), there could be a small depression that presents differently in transverse and longitudinal directions. Your numbers may reflect light loading, before such effects manifest themselves. Based on your data it does looks like we will need to confirm the Mu values under a range of loads.

Does anyone know whether the regolith is over carpet?

Good catch.